1. Field of the Invention
The present invention relates to a method for selectively hydrogenating a living polymer having olefinic double bonds. More particularly, the present invention is concerned with a selective hydrogenation method of living polymer having olefinic double bonds, which is superior in yield, selectivity and reproductivity.
2. Description of the Prior Art
Polymers with olefinic double bonds are usually used as elastomers. They may be utilized without any modification or in a modified state, for example, a vulcanized state.
However, the double bonds make the polymers poor in weather resistance, thermal resistance, oxygen resistance and ozone resistance, limiting the use range of the polymers.
This problem has been overcome by hydrogenating the double bonds of the polymers. Generally, the hydrogenation for the olefinic double bonds in polymers is carried out in the presence of a heterogeneous catalyst or a homogeneous catalyst.
A typical heterogeneous catalyst consists of a catalytically active ingredient and a support. The active ingredient, such as platinum and palladium, is impregnated into the support, such as carbon, silica and alumina. For the homogeneous catalyst, a catalytically active ingredient, such as nickel, cobalt and titanium, is mixed with an organometal compound serving as a reductant, such as organoaluminum, organomagnesium and organolithium. Relative to the homogeneous catalysts, heterogeneous catalysts are lower in hydrogenation activity and need a higher temperature and pressure for catalytic reaction. In the case of polymers, the heterogeneous catalysts have more serious difficulty in dehydrogenating them because of the high viscosity of the reaction system and the steric hinderance of the polymers. Thus, the heterogeneous catalyst is economically unfavorable not only because a large quantity of the catalyst is required for effective hydrogenation, but also because the high temperature and pressure causes the polymers to be decomposed and gelled.
In contrast, the homogeneous catalysts are very advantageous by virtue of high reactivity and hydrogenation efficiency even at a low temperature and pressure. However, it is difficult to obtain a product with high yield since the hydrogenation has a tendency to depend sensitively on the reduced state of the homogeneous catalyst. Further, a trace of impurities in the reactants causes the catalytically active ingredients of the homogeneous catalysts to easily lose their activity and thus makes it very difficult to control the hydrogenation efficiency and reproductivity in the reaction.
There are many techniques for the hydrogenation or selective hydrogenation of conjugated diens.
U.S. Pat. Nos. 3,644,588, 3,868,354, 3,541,064 and 3,700,633 disclose catalyst systems for hydrogenating or selectively hydrogenating ethylenically unsaturated polymers or ethylenically unsaturated aromatic copolymers, in which the metals of group VIII on the Periodic Table, especially, nickel or cobalt, are combined with the metals of groups IA, IIA and IIIB on the Periodic Table, especially, lithium, magnesium and aluminum alkyl as reductants.
U.S. Pat. No. 4,501,857 discloses that at least one bis(cyclopentadienyl)titanium compound and at least one hydrocarbon lithium compound are required for the selective hydrogenation of olefinically unsaturated polymers.
Similar to the just above-cited patent, U.S. Pat. No 4,980,421 suggests alkoxylithium compounds as a promoter, instead of hydrocarbon lithium compounds. In this patent, such a promoter may be prepared by directly adding an alcohol or phenol compound to living polymers or it may be the reaction mixture of organolithium compound and alcohol and phenol compound. It is also mentioned, that the hydrogenation system utilizing such a catalyst, is superior in reactivity under mild conditions. However, although reaction stability is accomplished by using alcohols and phenols as promoters, they may deleteriously affect the control of the molecular weight of the polymer upon carrying out continuous processes and the alcohols and phenols remaining unchanged into alkoxylithium may act as a poison against the catalyst activating species.
Another hydrogenation of the polymers having olefinic double bonds is described in U.S. Pat. No. 4,673,714. It is mentioned that the hydrogenation can be easily accomplished by employing bis(cyclopentadienyl) titanium diaryl compound only as a catalyst. This technique is allegedly very effective because it does not need additional alkyllithium compound as a promoter.
In U.S. Pat. No. 5,039,755, a hydrogenation reaction is carried out by adding hydrogen to a living polymer prepared with an alkyllithium initiator, deactivating the polymer under high hydrogen pressure and hydrogenating it in the presence of a titanium catalyst.
The above-cited patents are disadvantageous in that it is quite difficult to simultaneously obtain the high activity of the catalysts and high selectivity of hydrogen addition because the molar ratio of Li/Ti should be controlled depending on the change in the molecular weights of the polymers or the mole numbers of the living polymers. Furthermore, in the case of using living polymers, their anion ions may cause intermolecular cross-links prior to, during and subsequent to the hydrogenation, resulting in the formation of a high molecular weight polymer. In addition, the anions may modify the reduced state of the catalyst, serving as a deactivator thereof.